Nowadays, mainstream liquid crystal display units display color images by illuminating a transmissive liquid crystal display panel having color filters from its backside with a backlight. Further, conventionally, CCFLs (Cold Cathode Fluorescent Lamps) using fluorescent tubes have often been used in backlights, but usage of mercury has been restricted because of environmental concerns. Accordingly, light-emitting diodes LEDs are taking the place of CCFLs using mercury as a light source. (For example, see Patent Document 1.)
Backlights for liquid crystal panels are roughly divided into two types, edge-lit and direct-lit, depending on the placement of a light source. The edge-lit type, which has a light guide plate provided directly behind the back of a liquid crystal panel and has a light source placed on one side of the light guide plate, is used mainly for relatively small liquid crystal panels for displays for cellular phones and notebook personal computers. Further, the direct-lit type, which has a light source placed directly behind the back of a liquid crystal panel, is better in light use efficiency than the edge-lit type and can be lighter. Accordingly, the direct-lit type is being used for large liquid crystal panels.
Further, direct-lit backlights using light-emitting diodes as a light source include those using white light-emitting diodes as a light source and those using light-emitting diodes that emit light of the three primary colors of red, green, and blue and obtaining white light by mixing the colors.
However, the backlights using light-emitting diodes as configured above, which are constantly turned on when the liquid crystal display unit is in use, in the same manner as the conventional backlights using CCFLs, are desired to further reduce power consumption. Accordingly, it has been proposed, as in Patent Document 2, to reduce power consumption by dividing a backlight into multiple sub units and control luminance on a sub-unit basis.
Further, generally, light-emitting diodes are semiconductor devices that vary greatly in both luminance and chromaticity. Accordingly, it is necessary to screen light-emitting diodes because their random usage impairs image quality because of great variations in luminance and chromaticity. For example, Patent Document 3 has been proposed as a method of economically using light-emitting diodes having variations.    [Patent Document 1] Japanese Laid-Open Patent Application No. 7-191311    [Patent Document 2] Japanese Laid-Open Patent Application No. 2004-191490    [Patent Document 3] Japanese Laid-Open Patent Application No. 2006-133708